1. Accretion in Binaries II
Classes of X-ray binaries
Low-mass (BH and NS)
High-mass (BH and NS)
X-ray pulsars (NS)
Be/X-ray binaries (NS)
Distinguishing NS versus BH binaries

2. X-Ray Binaries Low mass: companion star mass less than one solar mass
High mass: companion star mass greater than one solar mass.

3. matter collects in wake
Wind Fed Accretion r
acc
The wind velocity is generally of the order of a few thousand km/s while the compact object moves at about 200 km/s. The speed of sound in the gas is about 10km/s, thus a bow shock forms around the compact object and this forms at r(acc). Capture will occur within a cylindrical region with its axis along the relative wind direction. Matter collects behind the compact in its orbital motion, in the wake.
An accretion disk might form if the accreted gas possesses residual angular momentum.
bow shock
matter collects in wake

4. Wind Fed Accretion
Matter in wind will accrete if its speed is less than the escape speed from the compact object at the radius of closest approach
Vw = wind velocity
Vx = velocity of compact object
Rc = capture radius

5. Luminosity If a neutron star binary has the following
parameters, Dorb=1012 cm, vw= 1000 km/s,
D(orb) is the distance between the compact object and the companion star.

6. Accreting Pulsars High mass X-ray binary
First discovered in X-rays, thus the name “X-ray pulsars”.
High mass X-ray binary

7. Accretion in magnetic field

8. Magnetosphere boundary
Magnetopheric boundary, rM, is where the magnetic pressure balances the ram pressure of accreted matter
Assuming spherical accretion and a dipole field with the dipole moment m = BSRS3, where RS and BS are the radius of the star and the field strength at the surface.
Used free-fall velocity for v and mass conservation to connect rho and m-dot.

9. Accretion Torque
For disc accretion, the radius of the magnetosphere is expected to be different from that for spherical accretion. It has been shown that the difference can be summarize by a multiplicative numerical factor.

10. Accretion Torque

11. Pulse Period Variations

12. What happens if the spin rate of the pulsar is faster than Keplerian rotation rate at the magnetospheric boundary?
Corotation radius lies outside magnetopheric boundary

13. Equilibrium Period
The accreted matter ceases to transfer angular momentum
to the neutron star when
Millisecond radio pulsars are likely descendents of weak field, spinning neutron stars in a binary system.
For a maximum (Eddington) luminosity and typical neutron star mass and radius, we have

14. Emission Geometry
Accreted plasma
Shock front
Hot spot
Settling matter

15. Pulse Profiles

16. Landau Levels Quantization of energy due to magnetic field:
where is the momentum of the electron parallel to the field,
n is the quantum number, and Bcrit is the critical field,
For B << Bcrit, the spacing between adjacent levels is

17. Cyclotron Lines

18. X-Ray Pulsar Cen X-3
Pulses are modulated at orbital period of 2.09 days

19. Distinguishing BH vs NS
M{x}: mass of the compact object
M{o}: mass of the companion star
i: inclination angle of the binary orbital plane
P{o}: period of the orbital motion
K{o}: maximum velocity of the companion star along the line of sight
Mass function

20. Compact object masses